Preload assessment in septic shock

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Background

The accuracy of intra thoracic blood volume (ITBV) as a preload index, instead of central venous pressure and wedge pressure, has been demonstrated by Lichtwark-Ashoff in mechanically-ventilated patients with acute respiratory failure [1,2]. The aim of our work was: (1) to verify ITBV as a preload index in patients suffering from septic shock and (2) to relate measured data (CVP, WP, ITBV, extra vascular lung water [EVLW]), with PaO₂/FiO₂ ratio.

Methods

15 patients suffering from septic shock were studied. Admission criterion was septic shock refractory to standard therapy with cathecolamines. All patients were monitored with a Swan-Ganz catheter and Cold system (Pulsion, Münich) and received a bolus of methylene blue (MB) at the rate of 3 mg/Kg. Hemodynamic and oxyphoretic data were measured before MB infusion, and 20 min, 1 h and 2 h after the end of MB infusion, so that 4 sets of data for each patient were obtained to give a total of 60 data points.

Results

There is a significant correlation between ITBV and stroke volume (SV) and between ITBV and cardiac index (CI). There is also a correlation between ITBV and EVLW. No correlation was found between WP and SV and CI. Then we plotted WP vs the PaO₂/FiO₂ ratio and found that when the PaO₂/FiO₂ ratio was <200 there was no difference in EVLW between patients with WP ≤16 mmHg and patients with WP >16 mmHg. On the contrary, plotting ITBV vs PaO₂/FiO₂, with PaO₂/FiO₂ ratio>200, EVLW was very significantly higher (P<0.001) if ITBV was >1100 ml/mq, than if it was ≤1100 ml/mq.

Conclusions

In septic patients, when a respiratory failure happens, ITBV can be normal or high (>1100 ml/mq). A high ITBV is related to a high EVLW, while a high EVLW with a normal ITBV means a normal preload. In this case, it is not useful to decrease preload to increase the PaO₂/FiO₂ ratio. ITBV together with PaO2/FiO2 ratio is useful to optimize hemodynamic therapy during respiratory failure in septic patients and allows us to identify the patients that need diuretic therapy.